1. Field of the Invention
The present invention relates to an optical bioimaging technology for biological samples such as small animals.
2. Description of the Related Art
In medical and biological fields, imaging of the distribution of molecular species in a living body is an important research technique. Heretofore, cellular-level imaging of molecular species has been widely performed using a microscope and a molecular probe labeled with fluorochrome or a chemiluminescent molecular probe. Hereafter, there is a demand for a device capable of observing the distribution of molecular species of interest in a living body not at the cellular level but at the level of organ or the entire animal body larger than cells. Such a device makes it possible to acquire images of a living body, such as a mouse, whose cancer cells are labeled with fluorescent probes every day or every week to monitor the growth of cancer cells of interest with the lapse of time. In a case where the growth of cancer cells in the body of an animal is observed by a conventional cellular-level observation device, it is necessary to kill the animal to stain a site to be observed or to attach a fluorescent substance to cancer cells. In this case, however, it is impossible to monitor the growth of cancer cells in the same animal body over a long period of time. For this reason, there is a demand for development of a device capable of observing molecular species in the body of a small live animal to obtain internal information of the small animal.
Near-infrared light relatively easily penetrates a living body. Therefore, light ranging from about 600 nm to 900 nm is used in devices for observing small animals. However, according to a conventional observation technique, a specimen is usually observed only from above, and cannot be simultaneously observed from various directions. Therefore, there is a case where, for example, when a mouse is observed from a specific direction, cancer is not detected, but when the mouse is observed from a direction opposite to the specific direction, cancer is detected. When a mouse is observed using an unidirectional observation device, an operator has no choice but to observe the mouse by a method approximate to multi-directional observation by picking up multi-directional images of the mouse rotated about its body axis by small increments. However, in this case, reproducible data cannot be obtained, and the mouse cannot be simultaneously observed from various directions. Particularly, in the case of observation of luminescence emitted from a living body, the intensity of luminescence is very weak, and therefore, it is usually necessary to perform integrated exposure on a two-dimensional detector for several tens of seconds to a few minutes. On the other hand, the intensity of luminescence changes with time, and therefore, when image pickup is performed every time the observation direction is changed, image pickup conditions are different among image pickup directions, and thus, resulting images are useless. For this reason, it is preferred that two or more images of a living body picked up from two or more directions can be simultaneously and parallely integrated on a detector for a long period of time. In the case of fluorescence measurement, light intensity is higher than that in luminescence measurement, and therefore, fluorescence images can be acquired in a relatively short period of time. Still, it is absolutely necessary to simultaneously pick up information from various directions to speedily obtain accurate data.
As a method for acquiring multi-directional images, one for sequentially acquiring images observed from various angles using a rotating reflection mirror in a time-sharing manner is known (see Patent Document 1). According to this method, a specimen can be observed from various directions by rotating the mirror and by changing the position of the specimen itself by parallel displacement, and therefore, it is not necessary to rotate the specimen or a two-dimensional detector.
However, the method disclosed in Patent Document 1 uses a rotating reflection mirror and therefore has the following drawbacks: a specimen is measured from various directions in a time-sharing manner, that is, multi-directional simultaneous measurement cannot be performed, and therefore, it takes a long time to complete measurement; images observed from different directions are picked up at different times, and therefore, measurement conditions vary according to time in the case of, for example, luminescence measurement because the intensity of luminescence changes with time; and a device using a rotating reflection mirror has a complicated structure.
On the other hand, as a method for simultaneously acquiring images observed from various directions, a method using a back-side mirror unit is known (see Patent Document 2). Patent Document 2 discloses a method for simultaneously acquiring multi-directional images of a sample constituting a 3D-image by picking up not only a front-side image but also a back-side image and a lateral-side image with the use of an imaging lens and two or more mirrors provided on the back side of the sample. However, in this case, the distance between the imaging lens and a virtual image formed by the mirror is larger than that between the imaging lens and the sample. Therefore, the imaging lens cannot simultaneously focus both direct light from the sample and light reflected by the back-side mirror toward the lens. In 3D-image acquisition, a focal point range, that is, the so-called depth of focus can be usually widened by stopping down a lens. Therefore, such multi-directional simultaneous measurement as disclosed in Patent Document 2 is achieved based on the premise that multi-directional images can be acquired even when virtual image is located at different distances of sample. That is, it can be considered that such a camera for acquiring 3D-image data is constructed on the premise that its lens is stopped down to increase the depth of focus.    Patent Document 1: U.S. Patent Application No. 20050201614    Patent Document 2: Japanese Patent Application Laid-open No. 2001-330915